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The Halo Theme was composed over the course of three days in the summer of 1999 by Martin O'Donnell. O'Donnell was asked by Joseph Staten to provide a soundtrack for the Halo MacWorld premiere demo. Staten told O'Donnell that the theme needed to evoke the game's "ancient, epic and mysterious" ambiance. To best convey this impression, O'Donnell decided to incorporate a Gregorian chant, which has since become a staple of the Halo series in its various forms.[1] He also drew inspiration from the Beatles' song "Yesterday", which helped him create the core melody.[2] O'Donnell has stated that he laid down the core melody of the theme in about half an hour. The theme was recorded with an orchestra and a choir one day prior to the MacWorld presentation.[1]

In its original incarnation, featured frequently in Halo: Combat Evolved, it placed heavy emphasis on deep, powerful drums and fast paced strings. The track is heard most prominently at the start of the level The Silent Cartographer, during the beach landing and during the final Warthog run segment of The Maw. The chant at the beginning of the piece was used for the main menu screen in-game. On the Halo: Original Soundtrack, however, it was the last track, and also included the bonus track Siege of Madrigal at the end.

For Halo 2, the theme was remixed into the MJOLNIR Mix. The basics of the theme remain the same, but it features electric guitar overlays by former Whitesnake guitarist Steve Vai. Halo 2 uses the track in its original form in certain parts, such as at the beginning section of the level Metropolis. The MJOLNIR mix is played at the end of Metropolis as John-117 boards the Scarab, and later during the end credits. It is the first track on the Halo 2 Soundtrack Volume 1.

The theme was again revised for Halo 3, this time recorded with a live orchestra instead of synthesized strings. The version used in Halo 3 finishes with the ending used at the end of The Maw, as opposed to the reprise of the opening chant used in the original theme and the MJOLNIR Mix. Occasionally, 343 Guilty Spark can be heard humming it with the IWHBYD skull activated in campaign.

The chant portion of the theme, now referred in the Halo 4 credits as "Halo Cantorum", is heard when Installation 03 is revealed in the level Composer. The main notes of the Halo Theme's chorus play subtly when Dr. Halsey tells her interrogator not to underestimate the Spartans. The same part is heard more prominently in the theme 117 when John neutralizes the external shield generators on Mantle's Approach.

The Halo Theme does not appear in Halo 3: ODST. While stated before release that the Theme would not appear in Halo: Reach,[4] several tracks in the soundtrack borrow motifs from it. For example, Immemorial from Overture takes three bars from the beginning of the chant, while Unreconciled from Tip of the Spear takes the drumbeat and uses both the beginning of the chant and the main melody.